Golden Week is a period that contains several holidays that fall within a seven day period. Coupled with luckily placed weekends, it is one of the busiest travel times in the country, coming after New Year's and obon.
The holidays are: the emperor's birthday, Greenery Day, Showa Day, Constitution Memorial Day, and Children's Day. There is also an unofficial floating holiday in there because of a unique rule; any day between two holidays is converted into a holiday for that year, and if a holiday falls on a Saturday or Sunday, the following Monday is a holiday for that year as well.
For those reasons, most companies give their employees the week off and some businesses shut down entirely. Most people travel and prices for hotels and airlines go up, it's an expensive time to go anywhere. And because everyone else is going places, it is crowded. Sometimes, the most popular destination spots, like Kiyomizudera, are swarmed with people.
I don't like traveling during this time because everyone else is doing it too.